The so-called limestone-gypsum wet-type flue gas desulfurization system, which is conventional and well-known, is designed to absorb sulfur oxides (hereinafter referred to as SO.sub.x or SO.sub.2) from an exhaust gas by using a calcium compound such as limestone or lime as an absorbent and to convert calcium sulfite, which is a reaction product, into stable gypsum which is recovered as a by-product. A desulfurizing reaction in this limestone-gypsum process is represented by the following reaction formula: EQU CaCO.sub.3 +SO.sub.2 +2H.sub.2 O+1/20.sub.2 .fwdarw.CaSO.sub.4.2H.sub.2 O+CO.sub.2
FIG. 63 illustrates a conventional flue gas desulfurization plant in which gypsum is recovered as a by-product by using limestone as an absorbent. An exhaust gas 101 is passed into an absorber tower 102 and brought into contact with a circulated slurry in a spraying zone 103 to become cooled, dedusted and desulfurized. Thereafter, the resulting gas is subjected to mist removal in a demister 104 and then discharged from the absorber tower 102.
On the other hand, a limestone slurry 117, which is used as the absorbent is supplied to a circulating tank 105 by a limestone slurry pump 110 and is then fed to the spraying zone 103 within the absorber tower 102 through a plurality of spray nozzles, fed by a circulating pump 108. The spray comes into contact with the exhaust gas 101, whereby the sulfur oxides in the exhaust gas 101 are absorbed. Then, the resulting slurry is returned into the circulation tank 105 for recirculation. The slurry 117, after absorption of SO.sub.x, is passed into a thickener 112 by a withdrawing pump 109, where it is thickened, and then the thickened slurry is stored in a gypsum slurry tank 113 and finally dehydrated by a centrifugal separator 115, whereby gypsum 116 in the form of a powder is recovered. A supernatant liquid 118 recovered in the thickener 112 and the centrifugal separator 115 is recirculated and reused for adjustment of scrubbing water and the limestone slurry within the system.
However, the prior art technique suffers from the disadvantage that the sprayed liquid slurry 117 is brought into contact with the exhaust gas flowing in a vertical direction in the absorber tower 102 or in the spraying zone 103 and hence, to insure a proper contact time, the height of the absorber tower 102 is increased, resulting in an increase in size and a complication in structure such as an increased length of upstream and downstream air ducts. Another disadvantage is that the capacity of the circulation pump 108 must be increased to match the increase in height of the absorber tower 102, resulting in an increased consumption of power. As the ducts become more complicated, the nonuniformity of gas flow within the absorber tower 102 is increased, resulting in a reduction in desulfurizing performance.
A system has been also proposed in which an absorbent circulating tank (pit) is connected to a horizontal absorber which is designed to spray the absorbing liquid horizontally. However, this system is not preferred, because the mist does not freely separate out, and the level of liquid in the bottom of the absorber tower varies, due to variations in amount of liquid sprayed and amount of exhaust gas, and thereby influences the gas flow. In addition, a system of a type in which an absorber and a circulating tank are separated, resulting in a necessarily increased length of the tower, has been proposed.
Accordingly, it is an object of the present invention to provide a wet-type flue gas desulfurization plant in which the length of the tower is reduced, and the apparatus is simplified.
It is another object of the present invention to provide a wet-type flue gas desulfurization plant which is self-supportable and in which the consumption of power is reduced.
It is a further object of the present invention to provide a desulfurizing plant which is economic and capable of achieving high desulfurizing performance.
It is a yet further object of the present invention to provide a wet-type flue gas desulfurization plant having high reliability, in which the absorber is compact, but still the amount of mist scattered can be reduced.
Further, it is another object of the present invention to provide a wet-type flue gas desulfurization plant in which the nonuniformity of exhaust gas flow is prevented, and spray pipes are firmly supported.
It is a further object of the present invention to provide a wet-type flue gas desulfurization plant in which performance of operation is enhanced by quality control of the absorbing liquid sprayed for contact with an exhaust gas, in which the quality of the produced gypsum is maintained, and in which the desulfurizing performance is enhanced.
It is a yet further object of the present invention to reduce the pressure drop within the absorber by efficiently removing the scattered mist and to reduce the size of the absorber to provide a high desulfurizing performance at reduced costs of equipment and operation.